1. Field of the Invention
The invention relates generally to a multiple exposure technology, and more specifically, to a mask set for double exposure process and the method of decomposing the mask pattern.
2. Description of the Prior Art
Optical lithography is a crucial step in semiconductor manufacturing. The basic principle of optical lithography is quite similar to that of chemistry-based photography. The images of the patterned photo-mask are projected through the high-precision optical system onto the wafer surface, which is coated with a layer of light-sensitive chemical compound, e.g. photo-resist. The patterns are then formed on the wafer surface after complex chemical reactions and follow-on manufacturing steps, such as development, post-exposure bake, and wet or dry etching.
The resolution of the photo-lithography system (R) can be described by the well-known Rayleigh's equation:
  R  =                    k        1            ⁢      λ        NA  in which λ is the wavelength of the light source, NA is the numerical aperture, and k1 is the factor describing the complexity of resolution enhancement techniques (RETs). As the very-large-scale integration (VLSI) technology pushes further into nanometer region, the feasible wavelength of the photo-lithographic system remains unchanged at 193 nm. Although there is anticipation that extreme ultraviolet lithography (EUVL) with the wavelength of 13 nm will replace traditional optical lithography, the availability of EUVL remains uncertain due to technical challenges and cost issues. On the other hand, the physical limit of dry lithography of NA is 1.0. The recently introduced immersion lithography has bigger NA (1.2), but it is harder to further increase NA to even higher values. Thus it is commonly recognized that resolution enhancement techniques (k1) remains a cost effective knob to achieve finer resolution.
As the gap between the required feature size and lithography wavelength gets bigger, the final wafer images are quite different from the patterns on the mask. In the past few years, resolution enhancement techniques (RETs) have become necessary in order to achieve the required pattern density. One well-known RET is the optical proximity correction (OPC), in which the mask patterns are intentionally “distorted” so that the desired image can be formed on the wafer. Nowadays, considerable amount of computing power has to be dedicated to these kinds of post-layout processes.
In addition to the above-mentioned OPC technology, a process of double patterning has been developed as a cost-effective way to further scale into the deep submicron domain, using the same lithographic technology. One popular form of double patterning is the double exposure lithography, wherein a given layout is split or decomposed into two sets of patterns, each of which is printed using a separate mask in a separate exposure step. The desired layout may be constructed by these two separate patterns.
The greatest advantage of double exposure lithography is that we can use available photo-lithography technology and tools to manufacture finer patterns with even higher density. However, there are still many process issues to overcome in practice. For example, the large overlay region of two decomposed patterns is prone to suffer film punching problems after the etching process. Furthermore, in the application of splitting one single comb pattern into two comb patterns, the tooth portion of comb patterns may have inconsistent or asymmetric corner rounding profile due to the different pitches between the teeth. Besides, for some conventional double exposure method, the resulting target photoresist pattern formed from the two decomposed patterns may lack necessary corner rounding features in connection positions of teeth and base portions due to the perpendicular intersection thereof, thereby introducing potential leakage issue.
Therefore, it is still necessary to improve the conventional pattern split method for double exposure process in order to overcome the above-mentioned problems.